Prior Art
Esaki and Tsu in U.S. Pat. Nos. 3,626,257 and 3,626,328, have described semiconducting devices having a superlattice structure in which there is a periodic variation in semiconductor composition. Theoretically predicted non-linear electron transport phenomena are assumed to arise from the periodic conduction band potential which is produced by constructing a crystal containing alternating ultrathin layers of two semiconducting materials with different band gaps.
The layered superlattice structure may be formed by either doping or alloying techniques. In doping, a superlattice structure is produced by epitaxially growing a semiconductor material such as germanium, which is periodically doped so as to produce alternating ultrathin layers having different conductivity types, viz. p or n.
The alternate technique is to grow ultrathin layers with varying alloy compositions. Alternating layers of Group IV element alloys such as Ge.sub.1-x Si.sub.x may be used where x alternates between two values which lie between 0 and 1. Group III-V alloys may also be employed. Thus, III.sub.1-x III.sub.x V alloys such as Ga.sub.1-x Al.sub.x As or In.sub.1-x Ga.sub.x As and III V.sub.1-x V.sub.x alloys such as GaAs.sub.1-x P.sub.x where x alternates between two values which lie between 0 and 1 are suitable semiconductor materials for the production of superlattices.
A system investigated extensively has been GaAs.sub.1-x P.sub.x where x alternates between 0 and 0.5. In this system, the respective layers 16A and 16B as shown in FIG. 1 are GaAs, with a lattice parameter a.sub.o = 5.65A, and GaAs.sub..5 P.sub..5, with a.sub.o = 5.55A. It has been found that the strain occasioned by the difference in lattice parameters causes a high density of dislocations to be formed. The dislocations may be characterized as either threading or misfit dislocations. Misfit dislocations lie strictly within the plane of mismatch, and their presence there accommodates some of the misfit and thus relieves elastic strain.
We have discovered that there are two types of misfit dislocations in GaAs/GaAs.sub..5 P.sub..5 superlattice structures produced on a GaAs substrate. Dislocations of the first type (I) occur at the interfaces between successive thin layers in order to accommodate the sudden change in lattice parameter there; those of the second type (II) occur in the interface between the superlattice and its substrate. They are due to the misfit between the substrate and the superlattice taken as a whole. The misfit of Type I is given by: EQU f.sub.I = (a.sub.A - a.sub.B /a.sub.SL) (1)
where a.sub.A and a.sub.B are the lattice parameters of the individual thin layers and a.sub.SL is the average lattice parameter of the superlattice. Misfit of Type II is: EQU f.sub.II = (a.sub.S - a.sub.SL /a.sub.SL) (2)
where a.sub.S is the lattice parameter of the substrate. If the thicknesses of the component layers A and B of the superlattice are approximately equal, EQU a.sub.SL = 1/2 (a.sub.A + a.sub.B) (3)
otherwise a.sub.SL is some average of a.sub.A and a.sub.B weighted according to thickness.
Threading dislocations propagate with a component of their orientation in the direction of crystal growth. The overwhelming majority of them are formed by a bending of misfit dislocations of Type I and II out of the mismatch plane. Since diffusion coefficients are greatly enhanced along dislocations, these threading dislocations can serve as diffusion "pipes" in which the abrupt changes in concentration of the As and P species necessary to produce the required periodic conduction band are greatly reduced. The diffusion "pipes" will effectively short out the potential barriers between adjacent layers and lead to an ohmic current-voltage characteristic.
Interfaces between parallel crystals in perfect contact are either coherent or incoherent. They are coherent if the crystals are elastically strained to bring their lattices into register at the interface and incoherent if the lattices are not in register. Disregistry indicates that misfit dislocations and therefore threading dislocations are present.
It is an object of this invention to grow coherent superlattice structures.
It is yet another object of this invention to grow a superlattice structure having abrupt changes in concentration between layers.
It is still another object of this invention to minimize or eliminate threading and misfit dislocations.